Magnetic toner powder applicator

ABSTRACT

A device for applying magnetic toner powder from an outer layer of a roller sleeve across a gap to a transfer sheet in response to a discrete external magnetic field pattern provided by a master source document. The device includes a magnetic field source non-rotatively positioned within the roller sleeve for providing a magnetic bias field extending over a sector of the sleeve to interact with the external magnetic field pattern to produce on the transfer sheet a toner powder reproduction of the field pattern, and another magnetic field source for providing a magnetic field for removing excess toner powder from the outer layer of the roller sleeve to enable a uniform distribution of toner powder to remain on the sleeve, the powder distribution when adjacent to background areas of the external field pattern within the sector being less than the distance across the gap.

United States Patent [191 Klaenhammer Nov. 19, 1974 1 1 MAGNETIC TONER POWDER APPLICATOR [75] Inventor: Bryan L. Klaenhammer, Hugo,

Minn.

[22] Filed: Aug. 13, 1973 [21] Appl. No.: 388,049

[52] US. Cl ll7/l7.5, 10l/D1G. 13, 101/426,

118/637, 317/262 A [51] Int. Cl G03g 13/00, B05b 5/02 [58] Field of Search 101/1 R, DIG. 13, 426,

101/114; 117/17, 17.5, 37 LE; 118/620, 621, 624, 637, 639; 317/262 R, 262 A, 3; 346/74 3,345,294 10/1967 Cooper 118/637 X 3,457,900 7/1969 Drexler 118/637 3,553,464 1/1971 Abe 118/637 3,570,453 3/1971 Nuzum 118/637 3,575,139 4/1971 Nuzum 118/637 3,641,969 2/1972 Hakanson 1l7/l7.5 3,648,656 3/1972 Ogawa 118/637 3,754,526 8/1973 Caudill 1l7/17.5 X

Primary Examiner-Robert E. Pulfrey Assistant Examiner-13. H. Eickholt Attorney, Agent, or Firm-Alexander, Sell, Steldt & DeLaHunt [5 7] ABSTRACT A device for applying magnetic toner powder from an outer layer of a roller sleeve across a gap to a transfer sheet in response to a discrete external magnetic field pattern provided by a master source document. The device includes a magnetic field source non-rotatively positioned within the roller sleeve for providing a magnetic bias field extending over a sector of the sleeve to interact with the external magnetic field pattern to produce on the transfer sheet a toner powder reproduction of the field pattern, and another magnetic field source for providing a magnetic field for removing excess toner powder from the outer layer of the roller sleeve to enable a uniform distribution of toner powder to remain on the sleeve, the powder distribution when adjacent to background areas of the external field pattern within the sector being less than the distance across the gap.

11 Claims, 3 Drawing Figures PATENTE HEY I 91974 1 MAGNETIC TONER POWDER APPLICATOR CROSS REFERENCE TO RELATED APPLICATIONS This application is related to a patent application entitled Data Accumulation System Providing Magnetic Toner Powder Recording, Ser. No. 284,071, filed Aug. 28, 1972, by Wayne M. Beebe and Richard E. Fayling.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention generally relates to magnetic printing methods for effecting the transfer of images represented by magnetic field patterns, and is specifically directed to the application of magnetic toner powder from an outer layer of a roller sleeve across a gap to a transfer sheet in response to a discrete external magnetic field pattern provided by a master source document.

2. Description of the Prior Art Throughout the history of the development of electrostatic and ferrographic copying and printing systems, a variety of techniques for applying toner powders in accordance with electrostatic charge or magnetic field image patterns have been used. One technique, using a magnetic brush, drags the toner powder across a surface on which the powders are selectively retained in accordance with the electrostatic charge or magnetic field patterns. Such direct contact with the surface produces smearing of the toner powder into the background area with a resultant decrease in image quality.

U.S. Pat. Nos. 3,455,276 and 3,643,629 disclose applicator devices in which toner powders are attracted to the outer surface of a rotatable roller sleeve by a permanent magnet assembly non-rotatably positioned within the interior of the sleeve. In these devices, the roller sleeve does not directly contact a transfer surface upon which the toner powder is to be selectively applied. However, a sufficient quantity of toner powder is always present such that the powder contacts all portions of the transfer surface adjacent to the outer layer of the roller sleeve. A separate means such as a doctor blade is provided to restrict the amount of toner powder attracted to the outer layer of the sleeve by the inte rior magnets. In the applicator devices described in these patents, the function of the interior magnets is to provide fields having high gradients for affecting the attraction of the toner powder to the outer layer of the sleeve. The applicator devices described in these two patents have not provided satisfactorily sharp and clean images when used for the specific toner powder application described hereinabove for imaging a transfer sheet in response to the magnetic field pattern provided by a master source document.

SUMMARY OF THE INVENTION The present invention provides an improved method and device for applying magnetic toner powder from an outer layer of a roller sleeve across a gap to a transfer sheet in response to a discrete external magnetic field pattern provided by a master source document. The master source document used with the present invention is magnetized to provide a discrete external mag netic field pattern of one polarity extending through said transfer sheet and a remaining background area of an opposite polarity. The improved device includes a magnetic field source non-rotatively positioned within the roller sleeve for providing a magnetic bias field extending over a sector of the sleeve to interact with the external magnetic field pattern to produce on the transfer sheet a toner powder reproduction of the field pattern, and another magnetic field source for providing a magnetic field for removing excess toner powder from the outer layer of the roller sleeve to enable a uniform distribution of toner powder to remain on the sleeve, the powder distribution when adjacent to background areas of the external field pattern within the sector being less than the distance across the gap. The toner powder is not indiscriminately applied to the entire surface of the transfer sheet, and as a result smearing of the image and retentionof toner powder in background areas is minimized. The device of the present invention thus achieves a remarkable improvement in image sharpness and contrast.

The roller sleeve is rotatively coupled to a nonrotating roller support. A major component of the magnetic bias field provided by the non-rotatable magnetic field source reinforces the one polarity of the field pattern and opposes the opposite polarity, thereby aiding the transfer of toner powder where the reinforcement occurs to produce a toner powder reproduction of the magnetic field pattern which is substantially free of toner powder deposition in the background area.

A master source document magnetized to have such a discrete external magnetic field pattern and a transfer sheet are received in substantial contact with each other to have at least a portion of the surfaces of the document and sheet parallel to the axis of rotation of the roller sleeve. While the roller sleeve is rotated, the contacted source document and transfer sheet are transported with respect to the sector of the roller sleeve over which the bias field extends such that the surface of the sheet which is away from the source document is transported parallel to and spaced across the gap from that portion of the outer layer of the sleeve within the sector by a predetermined minimum distance. The source of magnetic field for removing excess toner powder from the outer layer of the sleeve enables a uniform distribution of toner powder to remain on the outer layer, said powder distribution when adjacent to background areas within said sector being less than the predetermined minimum distance of separation across the gap.

Preferably the magnetic bias field extending through the sector of the sleeve is centered about a gap defined by the predetermined minimum distance of separation and the radial intensity of the bias field gradually decreases in intensity away from the gap, such that the intensity thereof at the angular extremities of the sector is insufficient to substantially affect the deposit of toner powder onthe transfer sheet. Thus the transfer of toner powder at extended portions of the sector is prevented and the smearing of toner powder already deposited is minimized.

In one embodiment, the predetermined minimum distance of separation ranges between 0.010 and 0.050 inches (0.25-1.25 mm).

In another embodiment, the outer layer of the roller sleeve comprises a smooth electrically insulating material having an outer adhesive surface to promote adherence of at least a monolayer of magnetic powder. The

electrically insulating nature of the surface is believed to support a limited electrostatic charge which holds a quantity of toner powder on the outer layer.

The device may further comprise a second source of magnetic field secured to the non-rotating roller support member, positioned within the roller sleeve and extending axially in relation to the sleeve for providing a uniform magnetic field extending radially through and along another sector of the sleeve to attract the toner powder to the outer layer of the sleeve.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-sectional view of an embodiment of the device of the present invention in which a contacted master source document and transfer sheet are supported on an outer surface of a drum and toner powder to be selectively attracted across a gap to the transfer sheet is provided on the outer layer of a roller sleeve;

FIG. 2 is an expanded cross-sectional view of the roller sleeve, the outer layer of which supports the toner powder; and

FIG. 3 is a cross-sectional view of another embodiment in which the document and sheet supporting roller is replaced by a laterally transportable flat plate on which the master source document and transfer sheet are secured.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the embodiment shown in FIG. 1, the toner powder applicator device of the present invention comprises a drum l2 and a roller 14 positioned with their axes, l3 and 15 respectively, parallel such that their outer peripheries are spaced apart, leaving a gap therebetween. A master source document 16 and transfer sheet 18 are received on the drum 12. The drum 12 has a ferromagnetic outer layer 20 of soft iron or steel to provide a magnetic flux return path, and is provided with positioning and supporting members (not shown) to support the master source document 16 and transfer sheet 18 on the outer layer 20. In one embodiment, the supporting mechanism allows the selective release of either the master source document or the transfer sheet 18 to allow the sequential positioning of additional transfer sheets adjacent a single master source document 16, to allow multiple copies to be produced, as well as the release of the master source document 16 and sequential positioning of additional master source documents. The drum 12 is supported by members (not shown) and coupled to a drive mechanism (also not shown) which drives the drum 12 at a uniform and controllable surface velocity.

The roller 14 supplies a source of toner powder to the gap separating the drum l2 and the roller 14. The powder is selectively transferred across the gap and deposited upon the transfer sheet 18. The roller 14 has on its periphery a sheet 22, the details of which are shown in FIG. 2. The inner supporting layer 24 is a tube of aluminum which is provided with an electrically insulating layer 26, such as a coating of orthosilicate. The insulating layer 26 allows an electrostatic charge to build up on its outer surface for enhancing the attraction of the toner powder thereto. To prevent the smooth outer surface of the insulating layer 26 from allowing the toner powder to slip toward the bottom of the roller 14 due to gravitational forces, thereby resulting in a nonuniform distribution of powder on the surface, an adhesive layer 28 is provided thereon to maintain the toner powder in a fixed location. In one embodiment, the layer 28 is pressure sensitive adhesive tape having the adhesive coating facing outward. It has further been found desirable to prime the adhesive surface 28 with a monolayer 30 of either toner powder or a combination of toner powder and fine iron filings, the fine iron having a particle size of 10 to micrometers. The adhered particles in the monolayer 30 do not transfer across the gap to the transfer sheet 18 and thereby provide the desired mechanical restraints and magnetic flux paths for the non-adhered toner powder.

The roller 14 is axially supported in a manner similar to thatprovided for the support of the drum l2 and is driven at a controllable and uniform velocity by a drive mechanism (not shown) such that the surface speed of the roller 14 is controlled relative to the surface velocity of the roller 12. A surface speed for roller 14 ranging between 2 and 6 times that of the surface speed of the drum 12 has been found preferable.

A bias magnet 32 is positioned on a non-rotating support (not shown) adjacent the interior surface of the sleeve 22, opposite the gap between the drum 12 and the roller 14. The bias magnet 32 is preferably constructed from a slab of a rubber bonded flexible permanent magnet construction such as Plastiform brand permanent magnet material manufactured by Minnesota Mining and Manufacturing Company, Saint Paul, Minnesota. The bias magnet 32 is positioned concentric with the inner surface of the sleeve 22 and extends along the axial length thereof, and is uniformly magnetized perpendicular to its surface, thus providing a substantially uniform radial field over an extended sector, resulting in a low magnetic field gradient in the gap between the two rollers. The resultant magnetic bias field has a radial polarity which reinforces the polarity of those areas 17 of the discrete external magnetic field pattern in the master source document 16 and opposes the oppositely polarized portion 19 constituting the background area. The magnetic bias field thus aids the transfer of toner powder in the areas where the reinforcement occurs and produces a toner powder deposit 21 on the transfer sheet 18 corresponding to the magnetic field pattern in the master source document 16.

A trough 34 containing a source of magnetic toner powder 42 is provided at the bottom of the roller 14 and extends along the length thereof. The trough 34 is further provided with an extended sidewall 36 to facilitate the return of excess powders into the trough. A coater magnet 38, also preferably constructed of a flex ible permanent magnet construction, is non-rotatably positioned adjacent the inner surface of the sleeve 22 opposite the trough 34 and extending along the length of the roller 14. The field provided by this magnet attracts magnetic toner powder 42 from the trough 34 to the outer layer of the sleeve 22. As the roller is rotated, powder on the outer layer of the sleeve 22 is removed from the influence of the coater magnet 38 such that gravity causes most of the powder to fall back to the trough 34 and a thin but nonuniform layer 43 of powder is retained due to combined electrostatic and mechanical forces. Upon further rotation of the roller 14, excess powder on the surface is removed by a sweeper magnet 40 which attracts loosely held powders from the surface toward the extended wall 36 of the trough 34, thereby resulting in a uniform distribution 45 of powder on the outer layer of the sleeve 22. The sweeper magnet 40 similarly extends along the length of the roller 14 and is also desirably constructed of a flexible permanent magnetic material. It is desirably secured so as to be periodically removed, thus releasing the powders attracted to the extended wall 36 and allowing their return to the bottom of the trough 34.

While the sources of magnetic field 32, 38 and 40 are all shown as permanent magnets, and are preferably constructed of a flexible permanent magnet material, the magnets may similarly be made of any permanent magnet construction, and may also be suitably energized electromagnets. The polarity of the bias magnet 32 is subject to the restraints imposed by the cooperative interaction with the magnetic polarities on the master source document 16. In contrast, however, the polarity of the magnets 38 and 40 is not critical, inasmuch as no intentional interactions occur. When the roller 14 is made sufficiently small such that the fields produced by the three magnets significantly interact, such polarities must be considered.

The magnetic toner powder 42 to be used with the device of the present invention is substantially the same as those toner powders designed for use with conventional electrostatic systems. A preferred toner powder is disclosed in U.S. Pat. No. 3,639,245. Such powders are generally spherical in shape to promote freeflowing characteristics, thereby forming a more uniform layer 45 on the outer layer of the sleeve 22. Particles having an average diameter of 5 to 6 micrometers have been found to be especially desired.

The transfer sheet 18 preferably is plain paper, and may be any other medium which is receptive to the toner powder. The preferred master source document 16 comprises a homogeneous sheet of particulate permanent magnet material in a flexible binder. The preferred document has a coercive force of not less than 1,000 oersteds and a maximum energy product of not less than 5 X 10 gauss-oersteds. Preferably the document includes Plastiform brand permanent magnet material, in which the permanent magnet material comprises barium ferrite particles oriented to have their easy axis of magnetization normal to the broad major surfaces of the document. The coercive force, maximum energy product and particle orientation of the magnetic field sources 32, 38 and 40 are preferably the same as those of the master source document.

With a master source document 16 and transfer sheet 18 in contact with each other and in place on the surface of the drum 12, the gap between the outer layer of the sleeve 22 and the outer surface of the transfer sheet 18 is desirably between 10 and 50 mils. (0.25-1.25 mm), with a preferred spacing of about mils. (0.5 mm). This spacing has been found optimum when used with a particular combination of magnetic fields intensities provided by the preferred master source document 16 and bias magnet 32 described hereinabove. Other spacings will be optimum for other geometries of bias magnetic field sources and ranges of field intensities.

Although the rotational velocities of the drum l2 and the roller 14 may be identical, when the roller 14 is rotated at a surface velocity greater than (i.e. from 2 to 6 times) that of the drum 12, more toner powder is available on the outer surface 28 within a given time frame for selective attraction across the gap and deposit in the areas 21, thereby providing a greater range of image contrast for a single rotation of the drum l2. Depletion regions 44 are thus left as a result of the transfer of powders, which depletion regions are refilled with powder upon the subsequent passage of that portion of the sleeve 22 past the toner supply trough 34. The image density, i.e. the amount of toner powder transferred to a given area is thus controlled by the relative rotational velocity of the roller 14 with respect to the drum 12.

In the embodiment shown in FIG. 3, the master source document 16 and transfer sheet 18 are secured to a planar supporting mechanism 46 for transporting the superimposed master source document 16 and transfer sheet 18 adjacent a roller 14 covered with magnetic toner powder 42. In this embodiment, a mechanism (not shown) transports the supporting member 46 along a transport path (indicated by arrow 47) at a uniform and controlled surface velocity. The supporting member 46 includes a sheet of soft iron 48 to provide a magnetic flux return path for the magnetic fields in the master source document 16. Brackets (not shown) are provided on the supporting member 46 for securing the master source document 16, and the transfer sheet 18 in superimposition with a major surface of the master source document 16. In one embodiment, a mechanism (not shown) is provided for passing successive transfer sheets adjacent a single master source document 16 to provide multiple copies. Similarly, a mechanism (not shown) may be provided for passing additional master source documents past the roller 14. The roller 14 of FIG. 3 is substantially the same as the roller 14 shown in FIG. 1, with the exception that the bias magnet 32 is in a different position in view of the position of the planar support member 46 with respect to the roller 14.

The use of magnets for holding toner powders as described in U.S. Pat. No. 3,643,629 should not be confused with the provision of the bias magnet 32. It has been found that when the magnetic field provided by the bias magnet 32 is in the same direction as the magnetic field provided by the discrete magnetic field pattern areas 17 in the master source document 16, and when the bias magnet 32 does not itself contribute large field gradients at such areas, a collimation effect occurs, thereby causing the toner powder to deposit as a solid pattern, leaving a clean background area surrounding the deposit. In contrast, when the magnetic field provided by the bias magnet 32 is opposite to the magnetic field provided by the pattern areas 17 of the master source document 16, the toner powder tend to deposit on the transfer sheet at the fringes of the areas 21 corresponding the discrete pattern areas 17.

What is claimed is:

1. A device for applying magnetic toner powder to a transfer sheet positioned adjacent to a master source document which is magnetized to provide a discrete external magnetic field pattern of one polarity extending through said transfer sheet and a remaining background area of an opposite polarity, said device comprising:

a roller sleeve rotatively coupled to a non-rotating roller support, said sleeve having an outer layer for carrying magnetic toner powder and for depositing said powder on a surface of said transfer sheet,

means non-rotatively coupled to the roller support for providing a magnetic bias field extending over a sector of said roller sleeve, wherein a major component of said magnetic bias field reinforces said one polarity of said magnetic field pattern and opposes said opposite polarity, thereby aiding the transfer of toner powder where said reinforcement occurs to produce a toner powder reproduction of said magnetic field pattern which is substantially free of toner powder deposition in said background area, means for receiving a said master source document and transfer sheet in substantial contact with each other to have at least a portion of the surfaces of said document and sheet parallel to the axis of rotation of said roller sleeve, means for transporting said contacted source document and transfer sheet with respect to said sector of the roller sleeve and for rotating said sleeve during said transporting, wherein the surface of said transfer sheet away from the source document is transported parallel to and spaced from the outer layer of the roller sleeve within said sector by a pre' determined minimum distance of separation; and

means for providing a magnetic field for removing excess toner powder from the outer layer of the sleeve to enable a uniform distribution of toner powder to remain on the outer layer, said powder distribution when adjacent to background areas within said sector being less than said predetermined minimum distance of separation.

2. A device according to claim 1, wherein said prede termined minimum distance of separation ranges between 0.25 and 1.25 mm.

3. A device according to claim 1, further comprising said outer layer of said roller sleeve having a smooth electrically insulating material provided with an outer adhesive surface to promote adherence of at least a monolayer of said magnetic powder to said surface.

4. A device according to claim 3, wherein at least a monolayer of magnetizable particles having an average dimension ranging between 10-100 micrometers is adhered to said outer adhesive surface to provide a slight abrasive and attractive action upon said toner powder carried thereon.

5. A device according to claim 1, further comprising a second magnetic field providing means secured to said nonrotating roller support member, positioned within said roller sleeve and extending axially in relation to said sleeve, for providing a uniform magnetic field extending radially through and along another sector of said sleeve to attract toner powder to said outer layer.

6. A device according to claim l, wherein said means for providing a magnetic bias field is positioned inside said roller sleeve.

7. A device according to claim 1, wherein said means for providing a magnetic bias field produces said field radially extending through said sector of the sleeve and centered about a gap defined by said predetermined minimum distance of separation, wherein the radial intensity of said bias field gradually decreases in intensity away from said gap, such that the intensity of said bias field at the angular extremities of said sector is insufficient to substantially affect the deposit of toner powder on a said transfer sheet.

8. A device according to claim 6, wherein said magnetic bias field providing means comprises a slab of a flexible permanent magnet material uniformly magnetized normal to the thickness thereof, which slab is nonrotatably positioned within said sleeve, adjacent to and concentric with the inner surface thereof, extending over substantially the entire length thereof and over an angular sector of about 9. A device according to claim 1, further comprising means for controlling the surface velocity of said outer layer of the sleeve to be between 2 and 6 times the surface velocity of said master source document and transfer sheet during said transporting.

10. A device according to claim 9, wherein said transporting means comprises a rotatably supported and driven drum having a ferromagnetic outer layer for receiving said source document in contact therewith.

11. A method for applying magnetic toner powder to a transfer sheet positioned adjacent to a master source document which is magnetized to provide a discrete external magnetic field pattern of one polarity extending through said transfer sheet and a remaining background area of an opposite polarity, said method comprising:

providing a roller sleeve rotatively coupled to a nonrotating roller support, said sleeve having an outer layer for carrying magnetic toner powder and for depositing said powder on a surface of said transfer sheet, providing a magnetic bias field extending over a sector of said roller sleeve which is non-rotatable with respect to said sleeve, wherein a major component of said magnetic field reinforces said one polarity of said magnetic field pattern and opposes said opposite polarity, thereby aiding the transfer of toner powder where said reinforcement occurs to produce a toner powder reproduction of said magnetic field pattern which is substantially free of toner powder deposition in said background area,

receiving a said master source document and transfer sheet in substantial contact with each other to have at least a portion of the surfaces of said document and sheet parallel to the axis of rotation of said roller sleeve,

transporting said contacted source document and transfer sheet with respect to said sector and rotating said sleeve during said transporting, wherein the surface of said transfer sheet away from the source document is transported parallel to and spaced from the outer layer of the roller sleeve by a predetermined minimum distance of separation, and

providing a magnetic field for removing any excess of toner powder from the outer layer of the sleeve to enable a uniform distribution of toner powder to remain on the outer layer, said powder distribution when adjacent to background areas within said sector being less than said predetermined minimum distance of separation. 

1. A DEVICE FOR APPLYING MAGNETIC TONER POWDER TO A TRANSFER SHEET POSITIONED ADJACENT TO A MASTER SOURCE DOCUMENT WHICH IS MAGNETIZED TO PROVIDE A DISCRETE EXTERNAL MAGNETIC FIELD PATTERN OF ONE POLARITY EXTENDING THROUGH SAID TRANSFER SHEET AND A REMAINING BACKGROUND AREA OF AN OPPOSITE POLARITY, SAID DEVICE COMPRISING: A ROLLER SLEEVE ROTATIVELY COUPLED TO A NON-ROTATING ROLLER SUPPORT, SAID SLEEVE HAVING AN OUTER LAYER FOR CARRYING MAGNETIC TONER POWDER AND FOR DEPOSITING SAID POWDER ON A SURFACE OF SAID TRANSFER SHEET, MEANS NON-ROTATIVELY COUPLED TO THE ROLLER SUPPORT FOR PROVIDING A MAGNETIC BIAS FIELD EXTENDING OVER A SECTOR OF SAID ROLLER SLEEVE, WHEREIN A MAJOR COMPONENT OF SAID MAGNETIC BIAS FIELD REINFORCES SAID ONE POLARITY OF SAID MAGNETIC FIELD PATTERN AND OPPOSES SAID OPPOSITE SAID OPPOSITE THEREBY AIDING THE TRANSFER OF TONER POWDER WHERE SAID REINFORCEMENT OCCURS TO PRODUCE A TONER POWDER REPRODUCTION OF SAID MAGNETIC FIELD PATTERN WHICH IS SUBSTANTIALLY FREE OF TONER POWDER DEPOSITION IN SAID BACKGROUND AREA, MEANS FOR RECEIVING A SAID MASTER SOURCE DOCUMENT AND TRANSFER SHEET IN SUBSTANTIAL CONTACT WITH EACH OTHER TO HAVE AT LEAST A PORTION OF THE SURFACES OF SAID DOCUMENT AND SHEET PARALLEL TO THE AXIS OF ROTATION OF SAID ROLLER SLEEVE, MEANS FOR TRANSPORTING SAID CONTACTED SOURCE DOCUMENT AND TRANSFER SHEET WITH RESPECT TO SAID OF THE ROLLER SLEEVE AND FOR ROTATING SAID SLEEVE DURING SAID TRANSPORTING, WHEREIN THE SURFACE OF SAID TRANSFER SHEET AWAY FROM THE SOURCE DOCUMENT IS TRANSPORTED PARALLEL TO AND SPACED FROM THE OUTER LAYER OF THE ROLLER SLEEVE WITHIN SAID SECTOR BY A PREDETERMINED MINIMUM DISTANCE OF SEPARATION; AND MEANS FOR PROVIDING A MAGNETIC FIELD FOR REMOVING EXCESS TONER POWDER FROM THE OUTER LAYER OF THE SLEEVE TO ENABLE A UNIFORM DISTRIBUTION OF TONER POWDER TO REMAIN ON THE OUTER LAYER, SAID POWDER DISTRIBUTION WHEN ADJACENT TO BACKGROUND AREAS WITHIN SAID SECTOR BEING LESS THAN SAID PREDETERMINED MINIMUM DISTANCE OF SEPARATION.
 2. A device according to claim 1, wherein said predetermined minimum distance of separation ranges between 0.25 and 1.25 mm.
 3. A device according to claim 1, further comprising said outer layer of said roller sleeve having a smooth electrically insulating material provided with an outer adhesive surface to promote adherence of at leaSt a monolayer of said magnetic powder to said surface.
 4. A device according to claim 3, wherein at least a monolayer of magnetizable particles having an average dimension ranging between 10-100 micrometers is adhered to said outer adhesive surface to provide a slight abrasive and attractive action upon said toner powder carried thereon.
 5. A device according to claim 1, further comprising a second magnetic field providing means secured to said nonrotating roller support member, positioned within said roller sleeve and extending axially in relation to said sleeve, for providing a uniform magnetic field extending radially through and along another sector of said sleeve to attract toner powder to said outer layer.
 6. A device according to claim 1, wherein said means for providing a magnetic bias field is positioned inside said roller sleeve.
 7. A device according to claim 1, wherein said means for providing a magnetic bias field produces said field radially extending through said sector of the sleeve and centered about a gap defined by said predetermined minimum distance of separation, wherein the radial intensity of said bias field gradually decreases in intensity away from said gap, such that the intensity of said bias field at the angular extremities of said sector is insufficient to substantially affect the deposit of toner powder on a said transfer sheet.
 8. A device according to claim 6, wherein said magnetic bias field providing means comprises a slab of a flexible permanent magnet material uniformly magnetized normal to the thickness thereof, which slab is non-rotatably positioned within said sleeve, adjacent to and concentric with the inner surface thereof, extending over substantially the entire length thereof and over an angular sector of about 90*.
 9. A device according to claim 1, further comprising means for controlling the surface velocity of said outer layer of the sleeve to be between 2 and 6 times the surface velocity of said master source document and transfer sheet during said transporting.
 10. A device according to claim 9, wherein said transporting means comprises a rotatably supported and driven drum having a ferromagnetic outer layer for receiving said source document in contact therewith.
 11. A method for applying magnetic toner powder to a transfer sheet positioned adjacent to a master source document which is magnetized to provide a discrete external magnetic field pattern of one polarity extending through said transfer sheet and a remaining background area of an opposite polarity, said method comprising: providing a roller sleeve rotatively coupled to a non-rotating roller support, said sleeve having an outer layer for carrying magnetic toner powder and for depositing said powder on a surface of said transfer sheet, providing a magnetic bias field extending over a sector of said roller sleeve which is non-rotatable with respect to said sleeve, wherein a major component of said magnetic field reinforces said one polarity of said magnetic field pattern and opposes said opposite polarity, thereby aiding the transfer of toner powder where said reinforcement occurs to produce a toner powder reproduction of said magnetic field pattern which is substantially free of toner powder deposition in said background area, receiving a said master source document and transfer sheet in substantial contact with each other to have at least a portion of the surfaces of said document and sheet parallel to the axis of rotation of said roller sleeve, transporting said contacted source document and transfer sheet with respect to said sector and rotating said sleeve during said transporting, wherein the surface of said transfer sheet away from the source document is transported parallel to and spaced from the outer layer of the roller sleeve by a predetermined minimum distance of separation, and providing a magnetic field for removing any excess of toner powder from the outer layer of the sleeve to Enable a uniform distribution of toner powder to remain on the outer layer, said powder distribution when adjacent to background areas within said sector being less than said predetermined minimum distance of separation. 